Cesare Casadei marked his Leicester debut with a stoppage-time winner after Aaron Ramsey had threatened to derail the Foxes' 100% start to the season.
Casedei, signed on loan from Chelsea on Wednesday, made an instant impact to make it four wins out of four for new Foxes manager Enzo Maresca. No Leicester side had previously managed such a feat at the start of a season.
Ramsey's first Cardiff goal since 2011 cancelled out a fine debut senior strike from Wanya Marçal-Madivadua, Leicester's 20-year-old Portuguese winger.
Cardiff, still winless in the Championship under new manager Erol Bulut, saw defender Mahlon Romeo sent off before the final whistle.
Leicester began as though they meant to continue a winning run that had seen Coventry and Huddersfield bested, plus an EFL Cup success at Burton's expense.
Cardiff might have snatched the lead when Ike Ugbo headed an early chance narrowly over after Ramsey had located him in front of goal but already the Bluebirds had been riding their luck.
First they saw Kelechi Iheanacho's curling effort bounce back off a post and referee Graham Scott take no action when Romeo appeared to barge Mavididi over in the box near the byline.
Leicester took the lead in the 36th minute when Cardiff failed to deal with a corner that bounced to Marçal-Madivadua some 15 yards from goal, and was fired into the net through a mob of players at extraordinary velocity.
Jak Alnwick, in the Cardiff goal, was forced into saves from Mavididi and Kienan Dewsbury-Hall before Ramsey levelled in first-half stoppage time.
The man who began his career as a Bluebird let fly from outside the box and Leicester goalkeeper Mads Hermansen was powerless to prevent the ball from arrowing into the top corner and Ramsey ran almost the length of the pitch to celebrate with the jubilant away corner.
The Wales midfielder has signed on for a third Cardiff spell in July, had not scored for them since 2011 in a 2-0 win that had also been against Leicester in a Championship fixture.
Ramsey was denied a second early in the second-half by a block by Jannik Vestergaard after Yakou Meite, on for Ugbo, had played him in. Hermansen then saved from Joe Ralls in the aftermath.
That roused Leicester and Alnwick had to make a good save to deny Callum Doyle, who went close with a deflected effort soon after.
Ramsey fired at Hermansen after the hour mark as Cardiff again suggested they might complete the comeback.
Meite, sent through by Romeo, should have done so with nine minutes remaining when Romeo sent him clear but the substitute shanked wastefully wide.
How wastefully became apparent in stoppage time when lashed home in the box after Dewsbury-Hall's shot had been blocked by Jack Simpson and rebounded to Jamie Vardy. Vardy went down seemingly fouled but referee Scott played on to allow the Italian to have the final word.
Scott then showed a second yellow to Romeo, for barging Dewsbury-Hall over off the ball.